Recently, synthetic quartz glass bodies including a quartz glass tube, ingot etc. which are materials for an optical fiber mother material and a heat treatment jig (core tube, wafer boat, etc.) used for a semiconductor production have been required high precision and high purity. Processes for fabricating optical fiber mother materials that have been used include: the outside vapor deposition method (hereinafter called the OVD method) that comprises rotating a columnar or cylindrical heatresisting substratum with a smooth outer peripheral surface, blowing and depositing quartz glass fine particles on the surface thereof, forming a porous quartz glass mother material and subsequently heating and transparently vitrifying the porous quartz glass mother material on the heat-resisting substratum or on the mold base inserted after the heat-resisting substratum is extracted, the vapor-phase axial deposition method (hereinafter called the VAD method) that involves depositing quartz glass fine particles in the axial direction of a starting rod to form a porous quartz glass mother material, which in turn is heated and transparently vitrified; the MCVD method; and a combination method of the above-described method. For a starting rod, a heat-resisting substratum, or a mold base used in the fabricatiing methods, materials including graphitized carbon as well as ceramics such as quartz glass, alumina, zirconia, mullite, silicon carbide, silicon nitride and boron nitride are used and, recently, carbon fiber-reinforced carbon composites (hereinafter called C/C composites) have come to be used.
As a large-sized optical fiber mother material with the lower cost is reqired, the large-sized optical fiber mother material for producing optical fibers has been attempted, but this requires large-sized porous quartz glass mother materials. Preparation of a large-sized, highly precise porous quartz glass mother material requires a starting rod, heat-resisting substratum or mold base that is more elongated with higher preciseness. When distortion, bend or the like of the starting rod, heat-resisting substratum or mold base occurs, it may create deflection in the center to disturb the shape formation of the porous quartz glass mother material, and during its transparent vitrification it may also produce a bend or distorton in the optical fiber mother material due to nonuniform heating because of the heating with rotation, which is intended to provide uniform heat from the heater, in that the mother material is heated with rotation so as to uniformly receive heat from a heater during its transparent vitrification. Furthermore, the large-sized optical fiber mother material increases the treatment temperature and imposes a large weight on itself, requiring further high load resistance and heat resistance in the starting rod, heat-resisting substratum or the mold base. Each of materials constituting the starting rod, the heat-resisting substratum and the mold base has different advantages and drawbacks. Quartz glass exhibits considerable heat resistance and also shows excellent processability as compared with ceramics etc. On the other hand, the increase in cost is significant for carry out the high level processing technology to achieve the extremely high level of preciseness, for example, in the case of preparation of a quartz glass mandrel with 50 mm or more in outside diameter and 5,000 mm or more in length. In addition, in the case where a number of short quartz glass rods are combined together to obtain an elongated quartz glass mandrel, this involves the welding of quartz glass rods. It is difficult to perform this operation while preventing bends at the welded portions, and this results in a cost increase. Even if an elongated quartz glass rod with high precision is obtained by means of the processing, care must be taken in the handling of the rods to prevent damage such as a fracture or cracks due to the nature of glass. Thus, in comparison with conventional small members, there are great increases in working difficulty and risk of breakage in the handling operation.
In the case where the above-described starting rod, heat-resisting substratum or mold base is made of ceramic, there are synergistically increased cost for production of the ceramic member, because such ceramics with highly purity and excellent heat resistance is very expensive, and has a difficulty in its processing so that the fabrication of such ceramics with high precision involves the grinding of a significant volume of the ceramics. In the case where the above-described starting rod, heat-resisting substratum or mold base is made of graphite, it is obtained with relatively low cost, and has an excellent heat resistance and processability and easiness in its handling. However, it lacks strength and it is thus difficult to achieve strength to withstand the load during the production or dehydration/transparent vitrification of a large porous quartz glass mother material. C/C composites that are recently proposed have problems for production such as synthesis or molding and problems for process precision. Because of these problem, the starting rod, heat-resisting substratum or mold base with lengths more than 1,000 mm and outside diameters more than 100 mm are very difficult to practically produce as a single member, and is therefore produced as an integrated member formed by joining a plurality of rod in series. The method used for the joining comprises perforating a hole 36 near the end of a rod and fixing by a pin 37, and joining the rod by pinching using a slit 38, as shown in FIG. 11. This joining method involves an excessive load on the pin and break it, resulting in incomplete connection and a loss of size precision. Accordingly, a desirable shape of an optical fiber mother material cannot be produced.
In addition, the production of a heat treatment jig used in semiconductor production also requires cost reduction. A conventional method so-called Bernoulli method, which melts and deposits a quartz powder while supplying it into an oxyhydrogen flame, cannot meet the requirement for large-sizing and high precision. Instead, there is proposed a method (hereinafter called the “mold melt method”) that involves disposing a mandrel in the center of a heat-resisting mold made of carbon etc., filling a silicon dioxide powder between the mandrel and the mold and subsequently melting and transparently vitrifying the powder in a heating furnace. This mold melt method allows the reduction of the work load and the number of jigs and tools for grinding and cutting as compared with a conventional method that includes machining quartz glass block to a desirable shape. In addition, the method provides easy molding and very high material yields, lead to effective cost reduction. Mandrels disposed in the center of a heat-resisting mold in the mold melt method includes ceramics such as quartz glass, alumina, zirconia, silicon carbide and silicon nitride, graphitized carbon (hereinafter simply called “graphite”), and further carbon fiber-reinforced carbon composites. In order to obtain a large-sized quartz glass body, it is preferable that a more elongated mandrel with a less diameter is used and filling with more amount of silicon dioxide powder is performed. However, melt vitrification of a large amount of silicon dioxide powder filled creates a large stress, causing damages of the quartz glass mandrel, and the reduction of mandrel diameter is also limited from the viewpoint of its handling. In the case of ceramics and graphite, there exist problems similar to the case for the starting rod, heat-resisting substratum or mold base used in the production of optical fiber mother material. Furthermore, in the case of a C/C composite member, the larger its size is, the more serious the production problems such as synthesis or molding of the material, or problems of processing precision are. As a result, practical use of a member with a length exceeding 1,000 mm is very difficult.
Therefore, a first object of the present invention is to provide a mandrel without the aforementioned drawbacks, which is used for producing quartz glass.
A second object of the present invention is to provide a method for manufacturing an optical fiber mother material using the aforementioned mandrel for producing quartz glass.
A third object of the present invention is to provide a method for producing an optical fiber by heat-drawing an optical fiber mother material using the aforementioned mandrel for producing quartz glass.
A fourth object of the present invention is to provide production of a large sized quartz glass body using the aforementioned mandrel for producing quartz glass.